1. Field of the Invention
The invention relates to a method of determining a temperature distribution of a part of an object, arranged in a substantially uniform, steady magnetic field, by means of magnetic resonance, which method includes the following steps: excitation of spins in the part and measurement of MR signals, containing location-dependent information of the excited spins, by means of MR imaging pulse sequences, determination of a phase image of the part from the measured MR signals, determination of a temperature distribution of the part from phase differences between the values of pixels of the phase image and the values of corresponding pixels of a predetermined reference phase image. The invention also relates to a device for carrying out such a method.
2. Description of Related Art
In the context of the present patent application a k-space is to be understood to mean a spatial frequency domain in which a path is traveled during the measurement of the MR signals by application of gradients to the steady magnetic field. The path in the k-space is determined by the time integral of the gradients applied during the interval between the excitation of the spins until the instant in time at which the MR signal is measured. The measured values of the MR signals corresponding to the most important part of the path or paths yield the inverse Fourier transformed values of an image of the object. Furthermore, gradients are to be understood to mean temporary magnetic fields which are superposed on a steady magnetic field and cause a gradient in the steady magnetic field in three respective orthogonal directions. Generally speaking, a gradient in the first direction is referred to as a read-out gradient, a gradient in the second direction as a phase encoding gradient and a gradient in the third direction as a selection gradient. Furthermore, a navigator gradient is to be understood to mean a gradient which is applied during the measurement of the navigator signal; a direction of the navigator gradient is referred to as the measuring direction.
A method of this kind is known from international patent application WO 25 94/23308. The known method is used, for example to determine a temperature distribution in a part of the human body. Such a part is, for example, a slice of the body which contains a tumor to be destroyed, said tumor being heated to a temperature beyond a limit temperature. In order to minimize the damage to other tissue of the body, it is necessary to have an accurate temperature distribution of the part available before and during the heating. This temperature distribution is formed by execution of the known method. For example, an ultrasonic source can be used to heat the tumor, the sound waves to be generated are then focused onto the part to be heated. In order to determine the temperature distribution of the part at a number of successive instants during the heating of the tumor, the reference phase image of the part to be heated is determined prior to the heating of the tumor, after which the MR signals of the part to be heated are measured by means of the MR imaging pulse sequences at the given instants during the heating. Subsequently, the phase image is reconstructed from the measured MR signals. The temperature distribution of the part to be heated is derived from the phase differences between the values of corresponding pixels of the reference phase image and the phase image. Subsequently, information derived from the temperature distribution obtained can be used to control the heating process, for example by determining the position of the heated location in the part from the temperature distribution.
It is a drawback of the known method that the temperature distribution contains errors which are caused by movement of the part of the body whose temperature distribution has been determined.